Some of the present reactors and systems include a reactor body having a substantially-planar bottom and one or more sidewalls extending from the bottom to define a recess, the reactor body defining inlet(s) and outlet(s) for liquid and gas, and a lid configured to be coupled to the reactor body to cover the recess such that the interface between the reactor body and the lid is substantially sealed, where at least one of the reactor body and the lid is configured to transmit incident ultraviolet light into the recess, and where the reactor body is configured to receive a photocatalyst in the recess such that at least a portion of liquid delivered to the recess through the liquid inlet(s) can react with the photocatalyst in the presence of the ultraviolet light to generate gas. Some reactors and systems include liquid and gas circulation systems having pumps and conduits.